Four good reasons not to fumigate your home for spiders

Spiders have your back. Photo by Andy Murray*

Artificial substances are not popular at the moment. The people of 2019 like things they deem "natural": wood, stone, kombucha, organic vegetables, those kinds of things. The mention of pesticides, additives to drinking water, or synthetic food sends shivers down spines, as people consider the possible long-term effects of using "unnatural" substances.

This fear of artificial chemicals has caused anaemia in Smarties and other previously brightly-coloured sweets.

I am amazed that, amongst this culture of filtering water and shunning potentially toxic chemicals, people are happy to fumigate their houses to keep away spiders. Led by popular belief, misunderstanding, and arachnophobia, people pay companies to kill all the spiders in their home, and prevent other spiders from thinking about entering.

Are you considering fumigating your home against spiders? If so, I hope the four points below are enough to discourage you.

1. It's bad for beneficial insects

Fumigation for spiders involves spraying your entire house with pesticide. In New Zealand, the major spider sprayers use pyrethroids, which target the nervous systems of animals, including spiders. Unlike pyrethrins, which are made by chrysanthemums, pyrethroids don't break down quickly in the environment. So you end up poisoning or repelling any insect inside and outside your house. Any insect. Including bees, butterflies, hoverflies, and other peaceful pollinators that you might otherwise encourage. Pyrethroids don't discriminate. Many pest control companies spray the exterior of your house to prevent spiders from re-entering after treatment; that means that pyrethroids will be floating around near your house, settling on the ground, and sticking around for a little while. Imagine being a bee and going to land on a bright, juicy flower - only to find that it's covered with a thin film of poison! Even if, like me, you don't reckon bees deserve special treatment, what have you got against the harmless and beneficial native insects that surround your home?

Don't make the butterfly sad.

2. It might be bad for you

Pyrethroids are more toxic than their plant-based counterparts. They are not selective, being toxic to insects, spiders, mammals, and fish. When pest control companies state that their treatments are 100% safe for you, your children, and your pets, they are telling porkies: not enough research has been conducted into the long-term effects of low-level pyrethroid exposure to say that it is completely safe. We do know that long-term effects of low-level exposure can include liver enlargement, slowed growth, and lower sperm quality (see here for a review). People who work with pyrethroids have been found to have decreased levels of antioxidant enzyme activity, which could possibly lead to illness. Rats exposed to pyrethroids as kits, and then as adults, were more susceptible to neurodegeneration than rats who were only exposed once, suggesting that the effects of exposure to pyrethroids are not always immediate. Long-term use of pyrethroids for spider control may therefore not be a good idea until we have more understanding of the consequences for humans. If you are trying to avoid exposure to toxins, synthetic pesticides, and other potentially harmful man-made chemicals, then you should not spray your home for spiders. However, pyrethroids are generally considered to be one of the safest pesticides for humans to use, and providing the proper precautions are taken, adverse effects from using them in your house are unlikely.

3. Spiders are useful

Spider control is a solution to a problem that doesn't exist. Spider control professionals often spread misinformation, and abuse their self-appointed positions as experts. Pest control companies make their living from the irrational fears of the general public, as they talk about "infestations" of spiders. Spiders are not generally social, they do not swarm, they do not carry human diseases. They are usually territorial, so their populations cannot get very dense. Unless you're a character in a movie, you cannot have an infestation of spiders.

What you can have is a few useful house guests. Spiders are so good at controlling insect pests that they have been suggested as biocontrol agents for greenhouses and arable land. The world's spiders eat between 400 and 800 million tonnes of insects and other invertebrates per year. The spiders in your house are helping you to control flies, other insects, or even other spiders. This doesn't mean that you won't ever see a fly, but there are probably fewer flies around your house than there would be if you had no spiders at all.

A spider infestation. Jumping spiders, like this one, do not make cobwebs. They patrol their territories looking for prey.

There are very few species of spider around the world that can make humans seriously ill with their bites. Those that frequently come into contact with humans include widow spiders (Latrodectus spp., such as redbacks and black widows), recluse spiders (Loxosceles spp.), male Sydney funnelwebs (Atrax robustus), and Brazilian wandering spiders (Phoneutria nigriventer). The last two are seasonal wanderers, meaning that they will only usually come indoors at particular times of the year, while searching for a mate. If you don't want them in your home, you can store firewood away from the house, declutter, and encourage spiders and birds that eat them. Catch (carefully!) any that you do see, using a cup and piece of card, and move them somewhere else. Kill them only if you feel you cannot catch them safely. Poisoning your entire house just to get rid of one spider is like spraying bleach on your entire lawn just to get rid of one weed. The deadliest of spiders is of no more risk to you than your dog or car, and no deaths from spider bites have been reported since antivenom was developed. If you're scared of spiders despite knowing they are not dangerous, then please see a doctor to help you treat your phobia, because if you try to rid your world of spiders you will lose. They are everywhere.

If you try hard enough, you may get bitten by this shy and lazy widow. Even if you are bitten, you may not experience any symptoms of envenomation. Note the hourglass marking, denoting potential ouchiness, though not all widow spiders carry one.

Many people spray their home for spiders because of the cobwebs, which not all spiders make, and in any case they are easily removed. I'm afraid that is just part of living in a world where spiders exist. Life is dirty. Pets leave pet hair, trees leave leaves, and everything makes dust. Cobwebs are a small price to pay for such effective pest control. However, if you really can't stand them, you could apply some repellent spray directly to the areas where you find webs, after you have moved the spider away. There is no reason to nuke your whole house.

4. It doesn't work for long

While pyrethroids persist in the environment for longer than their botanical counterparts, they do break down with exposure to sunlight and the micro-organisms in soil and water. While some of the residue in the soil may take a couple of months to degrade, the spiders will find a way in. Many non-native spiders can't survive outside during the winter. They're not trying to annoy you - they need to live indoors, and they can't find an alternative. I could not find any studies that say how long spider fumigation lasts, but I could find pest control companies that guarantee their treatment for up to a year. What do they base that certainty on? Not science. The cynical part of me suspects that the guarantee period is based on the placebo effect. When male spiders reach adulthood in the late summer/early autumn (the time of year when people start seeing more spiders, because they are looking for mates), people who were told the previous autumn that their house was spider-proof for a year may understandably assume that the treatment has suddenly worn off after a year of holding spiders at bay. They won't notice spiders until autumn, because they won't be looking for them, and there are fewer spiders walking around indoors in the warmer months.

A cheaper solution would be to move all the spiders you can find outside, or into outbuildings, and then put horse chestnuts (conkers) into the corners of every room of your house. There is no evidence that horse chestnuts repel spiders, but many people find that they do (this is also due to the placebo effect).

Or you could just learn to appreciate spiders. Then you will spend less time worrying about things that really do not matter.

*Andy didn't want the queen wasp to die. It's nice that wasps have friends too. But in New Zealand, Vespula wasps such as this one are extremely noxious pests that out-compete (and kill) native wildlife. They are also aggressive and dangerous, unlike spiders.